When manufacturing microelectronic devices, a phosphosilicate glass (PSG) layer is commonly deposited prior to forming metal wiring thereon. More particularly, the phosphosilicate glass layer can be used to reduce the steepness of steps on the device thereby enhancing the step coverage of the metal wiring. When used to reduce the steepness of steps, the phosphosilicate glass layer is typically reflowed prior to depositing the metal.
If the phosphosilicate glass layer is formed at a low temperature, however, a porous open structure of Si--O--Si having a large bond-angle and large pores may be formed. Accordingly, the step of reflowing the phosphosilicate glass may cause P.sub.2 O.sub.5 to dissipate from the PSG layer because of the high vapor pressure of P.sub.2 O.sub.5. A surface portion of the PSG layer may thus have a lower concentration of phosphorous than a lower portion of the PSG layer causing the surface portion to have a lower etching rate than the lower portion.
As shown in FIGS. 1A and 1B, a contact hole formed in the reflowed PSG layer using a wet etch may include a negative slope (or lip) at the surface portion of the PSG layer. This negative slope (or lip) may cause difficulties during later processing steps.